The present invention relates to the production of polyethylene glycol modified polyester fibers. The present invention also relates to the manufacture of yarns and fabrics from these copolyester fibers.
Polyester filament is strong, yet lightweight, and has excellent elastic memory characteristics. Polyester fabric resists wrinkles and creases, retains its shape in garments, resists abrasions, dries quickly, and requires minimal care. Because it is synthetic, however, polyester is often considered to have an unacceptable appearance for garment purposes when initially formed as a filament. Accordingly, polyester filaments require texturing to produce acceptable characteristics of appearance, hand, and comfort in yarns and fabrics. Even then, polyester is often viewed unfavorably in garments.
In pursuit of improved polyesters, various chemical modifications have been attempted to obtain desirable textile features. Unfortunately, some such treatments can produce unexpected or unwanted characteristics in the modified polyester. For example, polyethylene glycol enhances certain polyester properties, such as dye uptake, but diminishes other properties, especially those melt phase characteristics that are critical to filament spinning. Consequently, manufacturers have found that significant fractions of polyethylene glycol in copolyester can complicatexe2x80x94and even precludexe2x80x94the commercial production of acceptable copolyester filaments. To gain commercial acceptance, modified polyesters must be compatible with commercial equipment with respect to meltspinning, texturing, yarn spinning, fabric forming (e.g., weaving and knitting), and fabric finishing. This need for processing compatibility through conventional equipment has constrained the development of innovative polyester compositions.
To overcome the limitations of polyester compositions, polyester fibers are often blended with other kinds of fibers, both synthetic and natural. Perhaps most widely used in clothing are blended yarns and fabrics made of polyester and cotton. In general, blended fabrics of polyester and cotton are formed by spinning blended yarn from cotton fibers and polyester staple fibers. The blended yarns can then be woven or knitted into fabrics.
Cotton, like polyester, has certain advantages and disadvantages. Cotton is formed almost entirely of pure cellulose. Cotton fibers are typically about one inch long, but can vary from about one half inch to more than two inches. Mature cotton fibers are characterized by their convolutions. Under a microscope, cotton appears as a twisted ribbon with thickened edges. Cotton is lightweight, absorbs moisture quickly and easily, and has a generally favorable texture (i.e., hand) when woven into fabrics. Cotton, however, lacks strength characteristics and elastic memory. Consequently, garments formed entirely of cotton require frequent laundering and pressing.
Blends of cotton and polyester fibers have found wide-ranging acceptance as they combine the desirable characteristics of each. Even so, there are continuing efforts to develop polyester filament, yarns, and fabrics that more closely resemble those of cotton, silk, rayon, or other natural fibers. One example is polyester microfibers, which are characterized by extremely fine filaments that offer exceptionally good aesthetics and hand, while retaining the benefits of polyester.
A need continues to exist, however, for enhanced polyester compositions that have properties similar to those of cotton and other natural fibers, while retaining the advantages of polyester. One such composition and method for producing the same is disclosed by Nichols and Humelsine in pending U.S. patent application Ser. No. 09/141,665 (Polyester Modified with Polyethylene Glycol and Pentaerythritol), which is commonly assigned with this application. U.S. patent application Ser. No. 09/141,665, which is incorporated entirely herein by reference, discloses a polyester composition that includes polyethylene terephthalate, polyethylene glycol in an amount sufficient to increase the wetting and wicking properties of a filament made from the composition to a level substantially similar to the properties of cotton, but less than the amount that would reduce the favorable elastic memory properties of the polyester composition, and chain branching agent in an amount that raises the melt viscosity of the polyester composition to a level that permits filament manufacture under substantially normal spinning conditions. Including significant concentrations of branching agents to increase melt viscosity, however, is sometimes undesirable because branching agents promote cross-linking. This reduces filament strength, which can lead to processing failures.
Therefore, it is an object of this invention to provide polyethylene glycol modified polyester filaments that possess favorable characteristics similar to natural fibers, yet retain the advantages of polyester. It is a further object of the present invention to provide a method of copolymerizing polyethylene glycol (PEG) into polyethylene terephthalate (PET) to achieve a PEG-modified polyester composition that is readily spun into filaments, wherein the presence of branching agents is nonessential.
As is understood by those of ordinary skill in the art, modifying conventional polyesters with polyethylene glycol can improve certain polyester characteristics, yet can adversely affect others. For example, adding polyethylene glycol to polyethylene terephthalate improves wetting and wicking, but slows melt-phase polymerization kinetics. It also depresses melt viscosity and renders the processing of such PEG-modified polyesters somewhat impractical in commercial polyester spinning operations.
Accordingly, in one aspect, the invention is a method of copolymerizing polyethylene glycol into polyethylene terephthalate in a way that retains the favorable properties of polyethylene glycol while attaining a high intrinsic viscosity. This facilitates the commercial spinning of the PEG-modified polyester using conventional spinning equipment. As will be understood by those having ordinary skill in the art, copolymerizing polyethylene glycol into polyethylene terephthalate is conventionally achieved by reacting ethylene glycol and either terephthalic acid or dimethyl terephthalate in the presence of polyethylene glycol.
In brief, polyethylene glycol, which typically makes up between about 4 percent and 20 percent by weight of the resulting copolyester, is copolymerized into polyethylene terephthalate in the melt-phase to a relatively low intrinsic viscosity (i.e., a viscosity that will not support filament spinning). The resulting PEG-modified polyester is then further polymerized in the solid phase until the copolyester is capable of achieving a melt viscosity sufficient to spin filaments. Although polyesters having lower intrinsic viscosities can be spun by employing lower temperatures, this is often impractical using conventional spinning equipment.
By introducing a solid state polymerization (SSP) step, the invention reduces the need to add branching agents, such as pentaerythritol, to increase the melt-phase polymerization rate and thereby achieve an intrinsic viscosity that facilitates the spinning of filaments. Although effective at increasing polymer viscosity, branching agents promote cross-linking. Cross-linking leads to relatively weaker textiles. In contrast, the present method achieves a copolyester that contains a significant proportion of polyethylene glycol without relying on branching agents to achieve a melt viscosity that is suitable for spinning filaments.
In another aspect, the invention is a method of spinning the modified polyester composition to form partially oriented yarns (POY). The resulting copolyester POY is particularly suitable for yarns and fabrics, either alone or in a blend with one or more other kinds of fibers. In yet another aspect, the invention is a method of spinning the modified polyester composition to form staple filaments, which can be drawn (and perhaps crimped), and cut into staple fiber. Staple fiber, in turn, can be formed into polyester yarns by employing conventional spinning techniques. In addition, textured and spun yarns can then be formed into fabrics, preferably by knitting or weaving, either alone or in a blend with one or more other kinds of fibers.
The foregoing, as well as other objectives and advantages of the invention and the manner in which the same are accomplished, is further specified within the following detailed description and its accompanying drawings.